In the preferred field of fabricating glass articles, it is known to use the infrared radiation emitted by the articles leaving the machine for inspection or quality control purposes in order to detect any defects on the surface or inside the articles. It is necessary to perform quality control on such articles in order to eliminate those articles that present defects that might spoil their appearance, or worse, that might constitute a real danger for a subsequent user.
In conventional manner, the forming machine is constituted by a plurality of cavities, each fitted with a mold in which the article takes its final shape at high temperature. At the outlet from the forming machine, the articles are conveyed so as to constitute a line on a transport conveyor causing the articles to travel successively through various treatment stations such as spraying and annealing stations.
It would be advantageous to identify any forming defect as early as possible on leaving the forming machine and prior to treatment in the various treatment stations so as to be able to correct the defect in the forming machine as soon as possible. In the state of the art, various solutions have been proposed for inspecting high temperature articles leaving a forming machine.
For example, patent EP 0 679 883 describes apparatus constituted by two infrared sensors disposed on either side of a conveyor conveying articles at the outlet from the forming machine. Each of those sensors generates a signal in response to the heat radiation emitted by the articles. If such a signal does not correspond to a predetermined model, the articles are considered as being defective. It should be observed that that detection technique consists, for each cavity, in storing the image of an article that is deemed to be good in order to act as a reference model.
Such apparatus does not give satisfaction in practice for reasons which are set out in particular in document DE 199 02 316. It should be observed that the distances articles travel between the various cavities and the sensor used for inspection purposes are very different. However the articles cool very quickly, such that the infrared radiation from each article differs greatly on going past the sensor. In document DE 199 02 316, it is specified that the infrared radiation from the articles going past the sensor can vary in a ratio of 1 to 10, with the ratio being a function of the cavity from which each article originates.
In an attempt to provide a solution to that problem, document DE 199 02 316 proposes analyzing the thermal profile of the articles as picked up by the infrared sensor in order to determine statistically for each cavity an estimated thermal profile which is compared with the measured thermal profile in order to detect whether an article is or is not faulty.
The technique described by that document requires a plurality of thermal profiles to be analyzed statistically and inevitably leads to an approximation that affects the quality of detection. In addition, such a method does not enable a direct comparison to be performed between articles coming from different cavities. Finally, that technique makes use of measurement signals that can become saturated, thereby harming the quality of detection.
In the state of the art, it is also known to adjust a sensor so that the measurement signal is never saturated regardless of the cavity from which an article originates. Nevertheless, insofar as the level of infrared radiation varies over a large range as a function of the originating cavity, the level of the signal delivered by the sensor differs very greatly from one cavity to another. Thus, the level of the measurement signal is very weak for articles coming from the cavities that are furthest from the measurement sensor. Under such circumstances, the signal-to-noise ratio is poor, thereby restricting the ability to detect defects and also harming the quality with which defects are detected. Furthermore, that technique, like other known techniques, involves the use, after the measurement has been acquired, of correction means (such as a reference model (EP 0 679 883) or statistical analysis (DE 199 02 316)) presenting a degree of approximation that is harmful to quality of detection.